In order to produce high recording densities in hard disk drives (HDDs) recording bit length and recording track width of a recording medium may be reduced, in one approach. In order to read data from the recording medium having the recording bits with the reduced track width, without substantial amounts of error, a track width of a read head sensor (referred to as “read head track width”) and a stripe height (a depth from an air hearing surface of the read head) may also be reduced. The track width and the stripe height of the read head may be approximately 15 nm at a recording, density of 2 terabytes per square inch (Tbpsi) and approximately 5 nm at a recording density of 5 Tbpsi.
The miniaturization of the read head sensor leads to a smaller volume of the magnetic body from which the read sensor is constructed. As a result, read noise is very undesirably large due to the increased magnetic instability caused by the demagnetization field of the magnetic body. In addition, miniaturization of the sensor reduces sensor sensitivity. Therefore, problems relating to degradation of the signal-to-noise ratio (SNR) and a significant increase in the error rate are also observable. The magnetic bodies forming the read sensor film may comprise an antiferromagnetic layer, a pinned layer, and a free layer. Improvements in the magnetic stability of these magnetic bodies would be very beneficial to magnetic head manufacturing and usage.
The sensor changes resistance by using a tunneling effect of electrons in an insulated barrier layer, which is referred to as tunneling magnetic resistance (TMR). The sensor film may comprise a seed layer, an antiferromagnetic layer on the seed layer, a pinned layer formed on the antiferromagnetic layer, an insulated barrier layer formed on the pinned layer, a free layer formed on the insulated barrier layer, and a cap layer formed on the free layer.
Using this construction, low-frequency noise below about 50 MHz has been observed. The low frequency noise appears to be generated by the sensor, but attempts to reduce this noise, and to guarantee the reliability of a hard disk drive (HDD) employing such a sensor, have been unsuccessful.
A magnetic moment of the pinned layer is pinned by an exchange coupling, force from the antiferromagnetic layer. Conventionally, a MnIr disordered film having atoms arranged randomly in the antiferromagnetic layer may be used. The miniaturization of the stripe height accompanying the miniaturization of the read head increases the demagnetization field of the pinned layer, and the pinned layer becomes unstable; therefore, sensor miniaturization and a stronger exchange coupling force become necessary.
A large increase in the exchange coupling force between the antiferromagnetic layer and the ferromagnetic layer of a L12 ordered alloy of Mn3Ir has been shown. K. Imakita et al., “Giant exchange anisotropy observed in Mn—Ir/Co—Fe bilayers containing, ordered Mn3Ir phase,” Appl. Phys. Lett., 85, 3812 (2004). L12 ordered Mn3Ir is an alloy of Mn and Ir with an ordered placement of atoms that has a structure that places Mn in the center positions of the faces in a face-centered cubic (fcc) lattice and places Ir in the corner positions. When a sputtering device is used to deposit film of MnIr at room temperature, a MnIr disordered film is obtained. When growing an L12 ordered alloy of MnIr, it has been reported that substrate heating, high gas pressure film deposition, and cold film deposition processes are required.
A film deposition chamber capable of high-temperature film deposition and a cooling chamber capable of cooling the substrate during the film deposition process, therefore, may be used to deposit the ordered film of Mn3Ir. The fabrication of an ordered alloy by hot film deposition has been attempted, and the fabrication of a L12 ordered alloy of Mn3Ir was confirmed. The results of X-ray diffraction measurements confirmed a degree of order representing the extent of the ordered degree from 0.15 to 0.30, and a substantial increase in the value of the exchange coupling constant, Jk, that represents the strength of the exchange coupling force between the antiferromagnetic layer and the pinned layer from the conventional 0.6 erg/cm2 to 1.0 erg/cm2. Simultaneously, the blocking temperature, Tb, which is the temperature characteristic, substantially increased from 250° C. to 320° C.
The film thickness of the antiferromagnetic layer was thinned from 60 Å to 40 Å and used to fabricate a prototype head because Jk and Tb were greatly enhanced by producing, a L12 ordered alloy of Mn3Ir. The results were that the exchange coupling constant Jk increased from 0.6 erg/cm2 to 0.9 erg/cm2, and the blocking temperature Tb was the same as the conventional 250° C.
In contrast to a conventional read head, and in spite of the large increase in Jk, the prototype read head exhibited the baseline variations in the read signal waveform and experienced instability of the read waveform. The baseline variations in the read signal waveform are referred to as random telegraph noise (RTN) and are randomly generated over time. A prototype read head having a thick antiferromagnetic layer was studied, and a correlation between the generation of RTN and the antiferromagnetic layer film thickness was confirmed. Therefore, it would be beneficial to reduce the RTN which causes read errors.